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Can somebody please explain what is ment by functional datum and manufacturing datum, with respect to dimentioning and tolerancing.

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In simplest terms, Functional Datums should have something to do with a part. E.g. a mating surface or a centerline of the bearing surface of a rotating shaft.

A Manufacturing Datum is a feature used to locate and secure a part for a manufacturing operation. A Manufacturing Datum can use the Functional Datums or would be an entirely different feature. For example, say you have a complex casting which needs additional Machining on it. In particular, it’s functional datum needs to be machined into the rough casting. In order to be consistent with locating and loading the rough casting into the machining fixture, Manufacturing Datums are defined. They are likely defined as datum target points on the casting with Basic dimensions. Your tooling will be designed to hold the part on those Manufacturing Datums in order to be able to access the Functional Datums which need machining If everything is well-designed and toleranced correctly, you should have a repeatable process for Manufacturing.

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